In an imaging apparatus, such as a digital camera or a digital camcorder, an imaging signal is detected by forming a subject image photographed with an optical system on an imaging element, and it has been known that a neutral density (ND) filter is used for regulating the light quantity of the apparatus. When the ND filter is used, for example, a diaphragm at the time of image taking is opened or a shutter speed is reduced, and hence the range of images that can be represented can be widened.
Meanwhile, an electrochromic (hereinafter sometimes abbreviated as “EC”) element utilizing an EC material whose optical absorption properties (a colored state and a light transmittance) are changed by the electrochemical redox reaction of the substance has been known. The EC element has been expected to find applications in a display element, a dimming window, and an optical filter because the element has, for example, the following features. The element has a transmittance in its decolored state higher than that of a liquid crystal element or the like, is not affected by polarization, and has a memory property. Inorganic EC materials each using a metal oxide, such as WO3, and organic EC materials including conductive polymers, such as a polythiophene and a polyaniline, and organic low-molecular weight compounds, such as viologen and an oligothiophene, have heretofore been known as the EC materials.
The element has also been expected to find an application in a variable ND filter that can control an optical density in a stepless manner through the use of any such EC material. The variable ND filter requires an EC element that becomes gray or black in its colored state because the filter requires uniform spectral characteristics in a visible light wavelength region. The absorption wavelength of an organic EC element can be controlled to be wide by EC material design, and the optical density thereof in its colored state can be increased as compared to those of an inorganic EC element. Accordingly, the organic EC element has a wide light quantity regulation range and is hence particularly promising as the application of the variable ND filter to be mounted on an imaging apparatus.
In the spectral characteristics of the ND filter, an improvement in accuracy is required for, for example, an improvement in sensitivity of an imaging element. In particular, in order that satisfactory color reproducibility and a satisfactory color balance may be maintained, spectral characteristics in the entirety of the visible light wavelength region ranging from about 400 nm to about 700 nm have been required to be uniform. The phrase “spectral characteristics are uniform” as used herein means that a variation between transmittances at respective wavelengths in a certain wavelength region is small, and hence a transmittance spectrum is flat. A wide light quantity regulation range (light transmittance regulation range) has also been required.
In PTL 1 (corresponding to Japanese Patent Application Laid-Open No. 2004-93687), there is a description of an EC element whose color in its colored state is a gray color through the use of an inorganic EC material. In PTL 2 (corresponding to Japanese Patent Application Laid-Open No. 2001-519922), there is a description of an EC element that displays a color selected in advance through the mixing of a plurality of organic EC compounds, and there is a description that a gray color can be formed as one of the colors selected in advance.